Summary:
The Lake Giles Iron Project consists of a series of banded iron formation (BIF) hematite and magnetite prospects.
The parts of the north-northwest trending Yerilgee greenstone belt covered by the Project tenements comprise a layered succession of Archean rocks. At the interpreted base of the succession is a sequence of high-magnesium basalt flows more than 1 km thick overlain by komatiitic ultramafic volcanic rocks with narrow interflow BIFs and in some cases, other sedimentary rocks (Svensson, 2012).
The iron ore mineralisation consists of secondary pisolite mineralization, primary magnetite mineralization associated with un-oxidized BIF and ultramafic rocks, and goethite-hematite mineralization associated with oxidized BIF.
The mineralisation at Moonshine and Moonshine North deposits is associated with primary magnetite mineralization hosted by banded iron formation (BIF). The multiple BIF units steeply dip 75° to 85° to the west and strikes approximately 320° and 335° respectively the units have an average thickness of 15m, over a strike length of 17 km. The BIF units frequently outcrop.
The outcropping geology of the project area is comprised of a combination of unaltered silica rich BIFs and altered, enriched haematite/goethite BIFs. Weathering has resulted in the leaching of the majority of the silica from the BIFs, thus producing a rock rich in iron and low in silica, near surface. Below the depth of oxidation (generally between 45 m and 90 m from surface), the BIF units are comprised almost entirely of ferrous/ferric Fe(II,III) iron, silica and small amounts of alumina with occasional incipient iron sulphides (predominantly pyrite).
The iron grades are generally normally distributed, as opposed to log-normally for the altered haematite/goethite BIF, with grades consistently between 20% Fe and 40% Fe. Macarthur believes the majority of the underlying BIF units have experienced minimal metamorphism beyond their original formation. A notable exception to this is a pocket of high-grade magnetite mineralisation, up to 15 m true thickness, continuous along strike for >200 m, and >60% Fe, located in the Moonshine North deposit. This pocket of high-grade magnetite mineralisation is interpreted to be the result of structural and geothermal alteration of the primary BIF fabric.
The local high-magnesium basalts and ultramafics do not have significant outcrops due to strong weathering, especially proximal to the BIF ridges.
Logged komatiite and ultramafic units are typically thin (<10 m true thickness) and strongly weathered near the surface and are only identifiable at depth through drilling. The ultramafics are usually found proximal to the hanging wall of the BIF units.
Serpentinised high-magnesium basalts form the bulk of the geology at Lake Giles, forming thick, continuous, fine to medium grained granular units, occasionally cut by minor quartz veins and hosting sulphidic shales, locally including several metres of massive iron sulphides. Mafic intersections of interest have been occasionally investigated for gold mineralisation, but no specific targeting for gold has been recorded.
Most of the interpreted local faults tend to be sub vertical shear structures, truncating or occasionally displacing BIF bodies.
Structural deformation within the main BIF packages is generally weak, forming gentle kink banding and box folding, although some sections are interpreted as showing intense recumbent folding with sub-vertical axial planes, such as the southern edge of Moonshine. The larger BIF bodies at Moonshine and Moonshine North have relatively consistent thickness and dip to depths of over 250 m from surface as tested by a number of drillholes, increasing confidence that the remainder of the BIF ridges at Moonshine behave in a similar way and are not truncated at depth by synclines or other structural mechanisms.
The magnetite is present in the fresh BIFs along with high quantities of silica. This is the primary unaltered form of BIFs at site and in general has not been subject to any significant later iron enrichment.
The base of the Complete oxidation weathering profile strongly plunges downward proximal to the BIF bodies, rapidly rising to a relatively shallow depth of 3–10 m in the mafic/ultramafic rocks were distant from BIF units. This shallow depth of weathering is only observed at a handful of locations. Majority of drillhole collars at the Project are situated close to BIF units, and the depth to the base of complete oxidation is logged to greater depths, compared to holes drilled distal to the BIF units.